Well I guess the fact that a pair of select fins I purchased before going to Weymouth last year were so far off that after a hour of sanding I still couldn't use them (I only noticed the poor fit at the event) and so I stuck with the stock fins.

The simple fact is that they should fit and you shouldn't have to sand a whole lot. I understand that a little bit of sanding being required isn't a bad thing, ensures a perfect fit, but the requirement of an electric sander and lots of time seems excessive???

Sanding your finhead... there was an intresting story in the dutch surfmagazine about that (written by some very fast guys)

It was about this (till it fits) :

1) start with the width of your finhead
2) make the top of your finhead bit round (like tectonics)
3) (if is still doesnt fit) sand the backside of your finhead till it fits properly.

I have seen people go very fast with a fin that wasnt very tight in the finbox...
I have seen people go very fast with a fin to deep in the finbox...
I have seen people with fins that where so tight in the box that the hard foam around the box damaged
I have seen people that need about 20 min for fitting fin properly (every time)

There is some middle in sanding where the fin will fit most boards (99%). THat is for me:

bit tight in my Carbon Art boards. (I once forgot the screws and did 65kmh/36kn and fin was still in finbox)
bit less tight in my Starboard Isonic 2006

The Rubber mallet has worked for me . Some of my customers have thought they needed to sand the fins l supplied because they were so tight. l am saying that chinook boxes are a good standard size, and other brand boxes may varie. Maybe l haven't come across a brand of fin that is way out, even select fins fit. Just try a gentle tap and see if things give. These are wave boards though.
You will learn more from questions than insults.

With an electric sander the process doesn't take too long so I'm not overly bothered the time, suffice to say I've another Falcon in the post to me.

My concern however is that you could sand too much off one side or do it at an angle resulting in a fin which is very precise in its design not having the expected laminar flow over it, although I doubt this would have any noticeable effect.